The cellulose comprises natural cellulose (with crystal structure of cellulose I) and regenerated cellulose (with crystal structure of cellulose II). The cellulose forms fine fiber structure by crystallization to a certain extent. The diameters of the fine fibers are various as the species of cellulose materials (wood, cotton, hemp, rayon and so on), usually in a range from several nanometers to tens of nanometers. The cellulose fibers in such sizes are usually called as cellulose nanofibers. However, in normal cellulose based materials, the nanofiber units are bound firmly because of the strong hydrogen bonding interactions among them, the preparation of the cellulose nanofibers is very difficult and special processes are needed.
Cellulose nanofibers have very high mechanical strength and large specific surface area. Composite materials with excellent properties may be obtained by adding the cellulose nanofibers into synthetic resins. Therefore, a lot of people have tried to prepare nanofibers from natural cellulose. Current solutions comprise:
1. Method of High Pressure Homogenizing
Adding water to cellulose and breaking it by a pulverizer until a suspension liquid is obtained. The fine particles are obtained from the suspension liquid by a high pressure homogenizer, in which the liquid is erupted from slim tubes under high pressure and hits onto the solid wall. The cellulose microfibers prepared by this method has been commercial available by Daicel Corporation. However, since there are problems such as long processing time, low production efficiency, insufficient dispersion, nano-scale particles may not be obtained in the method of the high pressure homogenizer.
2. Method of Rotary Breaking
Machines used in this method are the ones precisely manufactured based on the refiners and beaters used in papermaking industries for breaking the pulp fibers into microfibers. The production efficiency of this method is high but the dispersion degree is not sufficient.
3. Method of Introducing Surface Charges
Cellulose is dispersed into water and is oxidized by TEMPO. The surface of the cellulose fibers is charged negatively by introducing carboxylic acid groups. The nanofibers are dispersed by electrostatic repulsion among the charges. This is disclosed by Isogai et al., see H. Fukuzumi, T. Saito, T. Iwata, Y. Kumamoto, A. Isogai, Biomacromolecules 2008, vol. 10, pp 162-165. The nanofibers thus obtained are applicable in field of packaging film. The problem of the method is that the procedures of oxidation and dispersion should be controlled accurately.
4. Method of Ball Milling
Cellulose and dispersant solvent are added into a ball mill pot with hard balls made of metal or ceramic. The cellulose is dissociated through the impact force of the hard balls from rotation and vibration of the ball mill pot. The dispersant solvent is normally water and may also be organic solvent. The problem of the method is that the dispersing efficiency is relatively low and it takes long time to dissociate cellulose to nanofibers. Furthermore, it was also proposed to recombine cellulose and synthetic resins by mix during ball milling, see T. Endo, R. Kitagawa, F. Zhang, T. Hirotsu, J. Hosokawa, Chemistry Letters, 1999, vol. 11, pp. 1155-1156. However, the size of fibers obtained by the mixing is in large scale not in nanometers.
In the above mentioned methods, it is easier to obtain the nano-scale fibers by the method of introducing surface charges. Since water is used as the dispersant in almost all the above methods, surfaces of the fibers thus prepared are bounded with hydroxyl groups, while carboxyl groups in the method of introducing surface charge, the fibers are hydrophilic. When adding the hydrophilic fibers to the synthesis resins, since the compatibility and contact therebetween are too bad to mix sufficiently, the reinforcement of the cellulose to the resins is not strong and water resistance of the prepared composite is weak.